1. Field of the Invention
The present invention relates to plethysmographic sensors and, more particularly, to a method and apparatus of calibrating plethysmographic sensors.
2. Description of the Related Art
Many plethysmographic instruments utilize plethysmographic sensors to measure blood flow, blood pressure, heart rate, breathing sounds, or other such functions, on the surface of the human body.
Examples of plethysmographic sensors or devices which use these type of sensors are disclosed in Baker, U.S. Pat. No. 4,781,200--Ambulatory Non-Invasive Automatic Fetal Monitoring System; Pfohl, U.S. Pat. No. 4,981,139--Vital Signs Monitoring and Communication System; Dickson, U.S. Pat. No. 4,458,687--Trans-Telephonic Acoustical and Electrical Heart Valve Monitor System; Kroll, U.S. Pat. No. 4,672,976--Heart Sound Sensor; Kroll, et al., U.S. Pat. No. 4,763,660--Flexible and Disposable Electrode Belt Device; Flowers, U.S. Pat. No. 4,258,720--Strain Gauge Plethysmograph; and Shirley, et al., U.S. Pat. No. 4,784,154--Interference Resistant Biomedical Transducer.
Plethysmographic sensors include acoustical pressure sensors.
Various problems occur in providing calibration of plethysmographic acoustical pressure sensors so that the calibration proves to be accurate and reliable when the sensors are applied in actual in-service conditions. Typically, the calibration of a plethysmographic acoustical pressure sensor loses its validity when the sensor is removed from the calibration facility and installed into actual in-service conditions. This problem generally occurs if the specific acoustic impedance of the plethysmographic acoustic pressure sensor is not substantially higher than the specific acoustic impedance of the calibration transmission medium and the specific acoustic impedance of the actual in-service transmission medium.
The transmission medium of most plethysmographic acoustic pressure sensors is human tissue, which may be assumed to have a specific acoustic impedance near that of water. If the specific acoustic impedance of the plethysmographic acoustic pressure sensor is not substantially higher than that of water, then the calibration must take place in a medium having a specific acoustic impedance comparable to that of water in order to match the environmental conditions of calibration to the environmental conditions of actual in-service use. This matching of environmental conditions poses a difficult problem for conventional calibration techniques.
Also, actual in-service conditions require that the plethysmographic sensors are mounted by a strap which is wrapped around the torso or a limb, or are applied independently. Therefore, the mounting condition of the plethysmographic acoustical pressure sensor in calibration should match the mounting conditions of actual service.
Conventional methods of calibrating plethysmographic acoustical pressure sensors include the acoustical coupler, the reverberation chamber, and the free field.
The acoustical coupler consists of a small rigid chamber with sealed ports to seat a sound source, a reference sensor, and the test sensor. The internal volume contains either a gas (air) or a liquid (water) and has dimensions which are much smaller than an acoustical wavelength, thus assuring uniform acoustical pressure throughout the volume. Acoustic pressures generated by the sound source are measured by the test and reference sensors and a comparison of the measurements provides the basis for the calibration. A calibration in a conventional acoustic coupler does not meet the mounting requirements of actual in-service conditions.
The reverberation chamber contains the same components as the acoustical coupler, except that the volume dimensions of a reverberation chamber are much larger than an acoustical wavelength and generally large enough to contain the test and reference sensors. Irregular reflections from the chamber walls generate a uniform sound energy density and insure that the test and reference sensors are subjected to the same sound pressure levels.
A free field calibration takes place in a very large volume (for example, the atmosphere, a lake, or an ocean) or in an anechoic chamber, such that the sound field is undisturbed by reflections. In this situation, a calibrated sound source generates a known sound pressure at a distance where the test sensor is located.
Accurate and reliable calibration in a reverberation chamber or free field in water is possible with the test sensor mounted similarly to in-service conditions. However, this procedure requires that the test sensor be submerged in water. This submersion may prove destructive since many plethysmographic acoustic pressure sensors are not hermetically sealed. As an example, Schanz and Schilling (Acustica, vol. 65, pp. 267-298 (1988)) describe a free field reciprocity method of calibrating PVDF-foil sensors, but the method requires the immersion of the sensor in a tank of water.
Further, an acoustical wavelength in water at, for example, 20 Hz is approximately 74 m and the required expanse of the free field must be several times an acoustic wavelength. Therefore, this method may prove impractical.